1. Field of the Invention
This invention relates to the conversion of liquid phase materials into vapor phase materials, and more particularly relates to the conversion of liquid phase materials into vapor phase materials in a cyclone evaporator for use in a variety of applications.
2. Description of Related Art
A variety of applications use vapor (gas) phase materials. One such application involves the fabrication of integrated circuit elements on a wafer in a chemical vapor deposition ("CVD") reaction chamber. To deposit materials on the wafer in the CVD reaction chamber, a composition of reactant gases and carrier gases is introduced into the CVD reaction chamber. The reactant gases chemically react to facilitate formation of the integrated circuit elements. In a growing number of CVD processes, one source of the reactant gas introduced into the CVD reaction chamber is an evaporated liquid precursor.
The liquid precursor may be either an organic or inorganic compound and has a characteristic vapor pressure that depends on the liquid precursor chemical composition. When depositing metal in a CVD reaction chamber, metal-organic liquids are generally used as the liquid precursors. The metal-organic liquid precursors typically have very low vapor pressures at room temperature and very fine decomposition points. Liquid precursors characterized by low vapor pressures have presented a significant challenge of increasing the vapor pressures of the liquid precursors to produce a controlled and economical quantity of vapor phase precursor ("gas precursor") while controlling liquid precursor evaporation rate and preventing decomposition.
The traditional way to evaporate liquid precursors is to use a bubbler such as those manufactured by Schumacher Inc. of Carlsbad, Calif. In the bubbler, the liquid precursor is maintained at an elevated temperature while a carrier gas is bubbled through it. As the carrier gas bubbles through the liquid precursor, a portion of the liquid precursor undergoes a transformation from liquid phase to vapor phase to form a gas precursor. The carrier gas entrains the gas precursor and transports it to the CVD reaction chamber.
Bubblers have several disadvantages. For example, the liquid precursor must be kept at elevated temperatures for extended periods of time by elevating the temperature of a surface element in proximate contact with a portion of the liquid precursor. The liquid precursor portion in proximate contact with the elevated temperature surface element often disadvantageously decomposes which denigrates the integrity of the liquid precursor. Furthermore, the gas precursor flow rate is a sensitive function of both the liquid precursor temperature and the remaining liquid precursor volume. Therefore, the liquid precursor volume must be monitored and the surface element temperature adjusted accordingly to maintain a steady flow of gas precursor.
A second approach to evaporating liquid precursor involves atomizing the liquid precursor and evaporating the droplets in a hot gas stream as discussed in an abstract by Richard Ulrich, et al, "MOCVD of Superconducting YBCO Films Using an Aerosol Feed System," Extended Abstracts, American Institute of Chemical Engineers 1994 Annual Meeting, Nov. 13-18, 1994, p. 16. However, this approach is unsuitable when used with low vapor pressure liquid precursors. When used with low vapor pressure liquids, the initial evaporation cools the droplet to a point where it has negligible vapor pressure (vapor pressure depends non-linearly on temperature). The gas precursor thus formed provides an insufficient quantity and an unsteady flow of gas precursor to efficiently and accurately sustain a CVD process.
A third approach to evaporating the liquid precursor involves flash vaporizing liquid precursor on a vaporization matrix structure at an elevated temperature as described in U.S. Pat. No. 5,204,314, by Kirlin, et al.